Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), phase change random access memory (PCRAM), and flash memory, among other types of memory.
Resistance variable memory devices, such as PCRAM devices, can include a structural phase change material such as a chalcogenide alloy, for instance, which can be programmed into different resistivity states to store data. The phase change memory cells are nonvolatile and the particular data stored in a phase change memory cell can be read by sensing the cell's resistance.
The chalcogenide alloy can exhibit a reversible structural phase change from amorphous to crystalline. A small volume of the chalcogenide alloy can be integrated into a circuit that allows the cell to act as a fast switching programmable resistor. This programmable resistor can exhibit greater than 40 times dynamic range of resistivity between the crystalline state (low resistivity) and the amorphous state (high resistivity), and is also capable of exhibiting multiple, intermediate states that allow multi-bit storage in each cell. That is, resistance variable memories may achieve multi-level cell (MLC) functionality via programming of memory cells to one of a number of different resistance levels.
In order to accurately read the programmed state of a phase change memory cell, it is beneficial to provide non-overlapping resistance distributions between the different program states. Programming a phase change memory cell such that the resistance of the cell is within the desired distribution can be challenging. For instance, fabrication processes and material variations among phase change memory cells can cause cells to reach a particular amorphous state or a particular crystalline state at different applied current/voltage levels. That is, the phase change material of different memory cells may undergo state transitions at different temperatures, which can result in widened resistance distributions among programmed cells and can lead to data read errors.